[PATCH v2 1/6] ipa: libipa: Add miscellaneous helpers

[Laurent Pinchart]

Hi Dan,

Thank you for the patch.

On Thu, Nov 07, 2024 at 10:25:03AM +0000, Daniel Scally wrote:
> We start to have functions that are effectively identical crop up
> across the IPA modules. Add a file allowing those to be centralised
> within libipa so that a single implementation can be used in all of
> the IPAs.
> 
> Signed-off-by: Daniel Scally <dan.scally at ideasonboard.com>
> ---
> Changes in v2:
> 
> 	- Dropped the Q(m, n) format helpers until they're needed.
> 	- \return statements after long description
> 	- Switched rec601LuminanceFromRGB() to return a double
> 
>  src/ipa/libipa/helpers.cpp | 77 ++++++++++++++++++++++++++++++++++++++
>  src/ipa/libipa/helpers.h   | 21 +++++++++++

Could we call this colours.{cpp,h} ? I'd like to keep libipa organized,
and add a generic "helpers" file only if we really can classify the
helpers in any meaningful way. I expect we'll get more colour-related
helpers in the future.

>  src/ipa/libipa/meson.build |  2 +
>  3 files changed, 100 insertions(+)
>  create mode 100644 src/ipa/libipa/helpers.cpp
>  create mode 100644 src/ipa/libipa/helpers.h
> 
> diff --git a/src/ipa/libipa/helpers.cpp b/src/ipa/libipa/helpers.cpp
> new file mode 100644
> index 00000000..6c038895
> --- /dev/null
> +++ b/src/ipa/libipa/helpers.cpp
> @@ -0,0 +1,77 @@
> +/* SPDX-License-Identifier: BSD-2-Clause */
> +/*
> + * Copyright (C) 2024, Ideas on Board Oy
> + *
> + * libipa miscellaneous helpers
> + */
> +
> +#include "helpers.h"
> +
> +#include <algorithm>
> +#include <cmath>
> +
> +namespace libcamera {
> +
> +namespace ipa {
> +
> +/**
> + * \file helpers.h
> + * \brief Functions to reduce code duplication between IPA modules
> + */
> +
> +/**
> + * \brief Estimate luminance from RGB values following ITU-R BT.601
> + * \param[in] r The red value
> + * \param[in] g The green value
> + * \param[in] b The blue value
> + *
> + * This function estimates a luminance value from a triplet of Red, Green and
> + * Blue values, following the formula defined by ITU-R Recommendation BT.601-7
> + * which can be found at https://www.itu.int/rec/R-REC-BT.601
> + *
> + * \return The estimated luminance value
> + */
> +double rec601LuminanceFromRGB(unsigned int r, unsigned int g, unsigned int b)
> +{
> +	return (r * .299) + (g * .587) + (b * .114);
> +}
> +
> +/**
> + * \brief Estimate correlated colour temperature from RGB color space input
> + * \param[in] red The input red value
> + * \param[in] green The input green value
> + * \param[in] blue The input blue value
> + *
> + * This function estimates the correlated color temperature RGB color space
> + * input. In physics and color science, the Planckian locus or black body locus
> + * is the path or locus that the color of an incandescent black body would take
> + * in a particular chromaticity space as the blackbody temperature changes.
> + *
> + * If a narrow range of color temperatures is considered (those encapsulating
> + * daylight being the most practical case) one can approximate the Planckian
> + * locus in order to calculate the CCT in terms of chromaticity coordinates.
> + *
> + * More detailed information can be found in:
> + * https://en.wikipedia.org/wiki/Color_temperature#Approximation
> + *
> + * \return The estimated color temperature
> + */
> +uint32_t estimateCCT(double red, double green, double blue)
> +{
> +	/* Convert the RGB values to CIE tristimulus values (XYZ) */
> +	double X = (-0.14282) * (red) + (1.54924) * (green) + (-0.95641) * (blue);
> +	double Y = (-0.32466) * (red) + (1.57837) * (green) + (-0.73191) * (blue);
> +	double Z = (-0.68202) * (red) + (0.77073) * (green) + (0.56332) * (blue);
> +
> +	/* Calculate the normalized chromaticity values */
> +	double x = X / (X + Y + Z);
> +	double y = Y / (X + Y + Z);
> +
> +	/* Calculate CCT */
> +	double n = (x - 0.3320) / (0.1858 - y);
> +	return 449 * n * n * n + 3525 * n * n + 6823.3 * n + 5520.33;
> +}
> +
> +} /* namespace ipa */
> +
> +} /* namespace libcamera */
> diff --git a/src/ipa/libipa/helpers.h b/src/ipa/libipa/helpers.h
> new file mode 100644
> index 00000000..51c74a36
> --- /dev/null
> +++ b/src/ipa/libipa/helpers.h
> @@ -0,0 +1,21 @@
> +/* SPDX-License-Identifier: BSD-2-Clause */
> +/*
> + * Copyright (C) 2024, Ideas on Board Oy
> + *
> + * libipa miscellaneous helpers
> + */
> +
> +#pragma once
> +
> +#include <stdint.h>
> +
> +namespace libcamera {
> +
> +namespace ipa {
> +
> +double rec601LuminanceFromRGB(unsigned int r, unsigned int g, unsigned int b);
> +uint32_t estimateCCT(double red, double green, double blue);
> +
> +} /* namespace ipa */
> +
> +} /* namespace libcamera */
> diff --git a/src/ipa/libipa/meson.build b/src/ipa/libipa/meson.build
> index e78cbcd6..ca4f3e28 100644
> --- a/src/ipa/libipa/meson.build
> +++ b/src/ipa/libipa/meson.build
> @@ -6,6 +6,7 @@ libipa_headers = files([
>      'camera_sensor_helper.h',
>      'exposure_mode_helper.h',
>      'fc_queue.h',
> +    'helpers.h',
>      'histogram.h',
>      'interpolator.h',
>      'lsc_polynomial.h',
> @@ -21,6 +22,7 @@ libipa_sources = files([
>      'camera_sensor_helper.cpp',
>      'exposure_mode_helper.cpp',
>      'fc_queue.cpp',
> +    'helpers.cpp',
>      'histogram.cpp',
>      'interpolator.cpp',
>      'lsc_polynomial.cpp',

-- 
Regards,

Laurent Pinchart